1. Field of the Invention
The present invention relates to a molded case circuit breaker and, more specifically to an arc chamber barrier of a molded case circuit breaker.
2. Background Information
Circuit breakers, including molded case circuit breakers, have at least one pair of separable contacts. A first contact is fixed within the molded case housing and the other contact, the “movable contact,” is coupled to an operating mechanism. Both contacts are disposed on conductive “arms” that are in electrical communication with either the line or load coupled to the circuit breaker. The operating mechanism is structured to move the movable contact between a first, open position wherein the movable contact is spaced from the fixed contact, and a second, closed position wherein the fixed and movable contacts are in contact and electrical communication. The operating mechanism may be operated manually or by the circuit breaker's trip mechanism. When a circuit breaker has multiple poles, each pole has its own set of separable contacts. Each set of contacts is disposed within a separate contact chamber within the molded case housing.
The housing, typically, has a base portion, in which the majority of components are disposed, and a primary cover. Both the base portion and the primary cover include a plurality of internal walls which define the contact chamber(s). Each contact chamber has an arc chamber portion and an operating mechanism portion. The operating mechanism, or part thereof, coupled to each contact arm is disposed in the operating mechanism portion. The arc chamber portion encloses the separable contacts as well as an arc chamber and, possibly, a slot motor. An arc chamber is structured to dissipate an arc following separation of the contacts. That is, when the contacts are separated an arc may form, especially during an over-current event. The arc is dissipated in the arc chamber but the arc still creates gases and possibly a spray of molten debris. A slot motor is a ferro-magnetic device structured to increase the speed of contact separation. That is, the slot motor is a loop of ferro-magnetic material disposed about the movable contact arm. Electro-magnetic fields within the circuit breaker effect the slot motor which in turn creates a magnetic field that increases the speed of the movable contact arm during separation. The stronger the electrical current, the stronger the magnetic field. Thus, during an over-current event, the magnetic field is strong and the speed of separation is increased. This configuration causes certain problems.
One problem is the blowback of arc gases and molten debris into the operating mechanism portion of the contact chamber. The arc gases are typically vented through an exhaust; however, the arc gases, which may be corrosive, and the molten debris also impact on the operating mechanism components causing damage thereto. Further, movement of the movable contact must be arrested prior to the movable contact or the movable contact arm impacting upon the slot motor or the housing. This is especially true during an over-current event when the movable contact arm is moving at great speed. Both of these concerns have been addressed by introducing an arc chamber barrier.
The arc chamber barrier is a non-conductive body shaped as an inverted “U” which is disposed between the arc chamber portion and the operating mechanism portion. The arc chamber barrier is typically fitted into the base portion of the housing and does not extend into the area occupied by the cover. Further, the size of the arc chamber barrier is slightly smaller than the size of the contact chamber so that it may be easily installed. When installed, the arc chamber barrier, which has two tines and a bight, is disposed over the medial portion of the movable contact arm so that the movable contact arm travels in the space between the tines. The bight is structured to arrest the movement of the movable contact arm. That is, the movable contact arm impacts the bight and is stopped. The arc chamber barrier body substantially fills the contact chamber. As such, debris is substantially blocked from traveling into the operating mechanism portion of the contact chamber. Further, because the gap between the tines is relatively small, the arc gases will typically pass through the exhaust. While arc chamber barriers are an improvement, the barriers create other disadvantages.
For example, a typical arc chamber barrier is not attached to the base portion of the housing. This lack of attachment along with the smaller size of the arc chamber barrier allows the arc chamber barrier to be moved, typically upwardly, when impacted by the movable contact arm. When the arc chamber barrier is moved from its original position, the arc chamber barrier may move into the path of travel of the movable contact arm, e.g. the tines may pinch the movable contact arm. Further, if the arc chamber barrier is not in the proper position, an additional impact from the movable contact arm may cause the arc chamber barrier to break and allow the debris therefrom to interfere with the operating mechanism. Also, because the size of the prior art arc chamber barrier is slightly smaller than the size of the contact chamber, the prior art arc chamber barrier allows arc gases and debris to pass around the arc chamber barrier.
Further, the prior art arc chamber barriers were structured primarily as a debris shield and not structured to optimize the separation speed of the movable contact arm. That is, in addition to other factors, such as, but not limited to, the strength of the magnetic field created by the slot motor, the impact speed of the movable arm during separation is controlled by the configuration of the circuit breaker housing and the operating mechanism configuration. One additional factor related to the configuration of the circuit breaker housing is the pressure within the contact chamber during an arc. That is, when an arc occurs and gasses are created, the pressure within the contact chamber increases. It has been determined that the greater the pressure, the greater the separation speed of the movable contact arm. In a three pole circuit breaker, the contact chambers extend parallel to each other. While the two outer contact chambers have a relatively small volume, the middle chamber is larger as it must accommodate additional components of the operating mechanism, such as, but not limited to, the operating handle. As such, the contact chamber of the “mechanism pole” of the circuit breaker has a larger volume than the contact chambers of the two outer poles. Thus, because the arc gases have a larger volume to fill, the increase in pressure within the mechanism pole contact chamber is less than the increase in pressure of the other contact chambers.
There is, therefore, a need for an arc chamber barrier structured to resist displacement when the separable contacts move between a second, closed position and a first, open position.
There is a further need for an arc chamber barrier structured to interlock with the circuit breaker housing cover.
There is a further need for an arc chamber barrier structured to fill a greater volume of the contact chamber thereby increasing the pressure within the contact chamber during an arc event.